Thermal analyses encompass an extremely wide spectrum of activities and a large variety of experimental methods. The key thermal property measured is the heat capacity of a sample of known mass, from which the specific heat of the sample material, which may be solid or liquid, can be calculated and basic thermodynamic functions such as phase transition enthalpies as well as reaction kinetics can be derived. Measurements of this type have been made in a very large temperature range from near absolute zero, i.e. 0 K. (Kelvin) to very high temperatures well above 1500 K. However, the overwhelming number of investigations in this branch of science is still restricted to one decade in temperature, namely the range from about 120 K. to about 1500 K.
Thermal analysis in the temperature range above 120 K. has seen a rapid development in instrumentation and automatization in the last ten to twenty years, while calorimetic experiments below 120 K. have been neglected somewhat and measurements in this temperature range are still a domain of a few cryogenic laboratories. The reason for this development of the art of calorimetric instruments is mainly due to the fact that the specific heat varies by many orders of magnitude in this temperature range.
A review of the state of the art of calorimetry is found in the book of W. Hemminger and G. Hohne "Grundlagen der Kalorimetrie" Verlag Chemie, Weinheim/New York, 1979. The definition of terms of this book are also used in this specification:
Twin calorimeter: A pair of measuring systems which are as equal as possible are operated symmetrically in a homogeneous environment. Measurement and reference samples are used which are as equal as possible with respect to their heat capacity, configuration, heat conductivity, heat transfer to the measuring system and so on.
Isoperibol operation: Isoperibol operation of a calorimeter means that the calorimeter is operated in an environment of a given, generally constant temperature from which the temperature of the measuring system differs as much as possible. The measuring system is coupled to the environment by a well defined heat conducting path or heat resistance of finite magnitude so that the heat exchange between the measuring system and the environment depends in a well defined manner only from the temperature of the measuring system and the temperature of the environment.
Scanning operation: The temperature of the measuring system of the calorimeter or the temperature of its environment is raised proportionally linearly with respect to time to time by means outside of the calorimeter and its environment.
Power compensated twin calorimeters (DPSC) are known which are operated in an isoperibolic scanning mode (Hemminger/Hohne, l.c. page 72, 73). These calorimeters comprise a pair of individual measuring systems in a common environment, the temperature of which is kept constant. The temperatures of the measuring systems are kept equal and raised linearly with time by individual regulated heating devices. The difference of the heating powers supplied by the heating devices to the individual measuring systems constitutes the measured magnitude.
Further differential or twin heat conductance calorimeters are known (Hemminger/Hohne l. c. page 181) which may be operated in an environment scanning mode (Hemminger/Hohne l. c. page 192, 193).